Field of the Invention
The invention relates to a compensation circuit for attenuating or eliminating undesired properties of an operational amplifier and to a corresponding method for attenuating or eliminating undesired properties of an operational amplifier.
A variety of embodiments of operational amplifiers have been known for many years. An ideal operational amplifier has an infinitely high input resistance over the entire frequency axis, no output resistance, an infinitely high amplification and a number of further ideal properties.
However, in reality, operational amplifiers are not ideal. The aforementioned properties deviate from the ideal case to a greater or lesser extent, and these deviations are frequently also non-linear.
It is particularly unfortunate here that a phase shift which depends on the frequency of the input signals occurs between the input signals and the output signal of the operational amplifier. This phase shift can have the effect that negative feedback from the output terminal of the operational amplifier to its inverting input terminal becomes positive feedback as a result of which the operational amplifier becomes unstable. In order to avoid this, the frequency response must be corrected. During this correction, it is ensured that the gain factor of the operational amplifier is reduced to values which are less than 1 for frequencies at which the phase shift assumes values an which can be critical for the stability of the operational amplifier.
For this purpose, the configuration and/or the external wiring of the operational amplifier must be changed, the change usually including the addition of a capacitor. One of the best known possible ways of performing a frequency response correction is known as Miller compensation.
FIG. 1 is a schematic representation of an operational amplifier with a compensation circuit which is used for frequency response correction. Here, the operational amplifier is designated by the reference OPAMP, and the compensation circuit is designated by the reference KOMP. The compensation circuit KOMP is formed in the example in question of a capacitor C0 whose terminals are connected to selected points inside the operational amplifier OPAMP.
The compensation circuit KOMP ensures that the operational amplifier is operated with a sufficiently high phase reserve, and thus operates in a stable fashion under all circumstances.
However, the compensation circuit KOMP also has effects on other parameters of the operational amplifier. The (compensation) capacitance of the capacitor C0 acts, inter alia, on the slew rate and the gain-bandwidth product of the operational amplifier.
The slew rate is generally proportional to I/C, largely independently of the topology of the circuit, and the gain-bandwidth product is proportional to Gm/C, I being the available current, Gm being the steepness of the transistors contained in the operational amplifier and C the compensation capacitance, also largely independently of the topology of the circuit. As the compensation capacitance C becomes larger, the slew rate and the amplification-bandwidth product therefore drop. This cannot generally be compensated by an increase in I or Gm because the technology is usually already operating at the limit of what is feasible.
Good (high) values for the slew rate and the gain-bandwidth product can therefore be achieved only if the compensation capacitance C is not too high. On the other hand, the compensation capacitance C must, however, not be too small because otherwise there is the risk of the operational amplifier becoming unstable.
For this reason, there is an optimum value or value range for the compensation capacitance C, and this value or value range should not, or must not, be significantly undershot or exceeded.
It becomes apparent that this is very difficult in particular if the capacitor is integrated into an integrated circuit which forms or contains the operational amplifier. The capacitances of the capacitors which are integrated into integrated circuits can in fact have very high tolerances (up to xc2x140%). Although the capacitors can be adjusted to the desired capacitance, for example by trimming with a laser, this process is very complex and expensive. A possible alternative to this is to provide the capacitor outside the integrated circuit which forms or contains the operational amplifier. However, it is then complicated and costly to use the operational amplifier.
Such problems and similar problems may occur not only in the case of frequency response correction but also when other undesired properties of the operational amplifier are attenuated or eliminated.
It is accordingly an object of the invention to provide a compensation circuit for attenuating or eliminating undesired properties of an operational amplifier and a method for attenuating or eliminating undesired properties of an operational amplifier which overcome the above-mentioned disadvantages of the heretofore-known circuits and methods of this general type and which reduce or eliminate the undesired properties of the operational amplifier as required in a reliable and easy fashion and which involve only little outlay.
With the foregoing and other objects in view there is provided, in accordance with the invention, a compensation circuit for attenuating or eliminating undesired properties of an operational amplifier, including:
a plurality of compensation circuit elements; and
at least some of the compensation circuit elements having at least one property selected from the group consisting of being selectively activatable and deactivatable, being connectable to the operational amplifier, and being connectable in different ways to respective other ones of the compensation circuit elements.
In other words, the compensation circuit according to the invention is defined by the fact that its elements can be activated and deactivated at least partly selectively and/or can be connected in different ways to other elements of the compensation circuit and/or to the operational amplifier.
As a result it is possible to ensure that the operational amplifier always operates in a fast and stable fashion as requested under all circumstances even if the elements which are required for compensation can be manufactured only with large tolerances. It is even possible to perform dynamic adaptation to the respective current conditions and requirements.
According to another feature of the invention, the operational amplifier has a given frequency response, and the compensation circuit elements correct the given frequency response of the operational amplifier.
According to yet another feature of the invention, a regulator is operatively connected to at least some of the compensation circuit elements, and the regulator configures the compensation circuit according to given specifications.
According to a further feature of the invention, the regulator configures the compensation circuit such that the operational amplifier operates in a fast and stable manner according to given requirements.
According to another feature of the invention, the operational amplifier has a transient response with a given duration and a given profile, and the regulator configures the compensation circuit as a function of at least one characteristic selected from the group consisting of the given duration and the given profile of the transient response of the operational amplifier.
According to yet another feature of the invention, the operational amplifier has input terminals, and the regulator determines the given duration and the given profile of the transient response from a ratio of magnitudes of voltages established at the input terminals of the operational amplifier.
According to another feature of the invention, the regulator operates based on the assumption that the transient response is still occurring if the voltages established at the input terminals of the operational amplifier are not of a substantially same magnitude.
According to a further feature of the invention, the regulator operates based on the assumption that the transient response has ended if the voltages established at the input terminals of the operational amplifier have substantially a same magnitude.
According to yet a further feature of the invention, the regulator operates based on the assumption that the operational amplifier is unstable if the voltages established at the input terminals of the operational amplifier alternately have substantially a same magnitude and different magnitudes.
According to another feature of the invention, at least some of the compensation circuit elements together define a given capacitance present in the compensation circuit, and the regulator reduces the given capacitance if the given duration of the transient response exceeds an upper limiting value.
According to yet another feature of the invention, at least some of the compensation circuit elements together define a given capacitance present in the compensation circuit, and the regulator increases the given capacitance if the operational amplifier is unstable.
According to a further feature of the invention, at least some of the compensation circuit elements together define a given capacitance present in the compensation circuit, and the regulator increases the given capacitance if the given duration of the transient response drops below a lower limiting value.
According to yet a further feature of the invention, the regulator adjusts the compensation circuit during a normal operation of the operational amplifier.
With the objects of the invention in view there is also provided, a method for attenuating or eliminating undesired properties of an operational amplifier, the method includes the steps of:
ascertaining given conditions selected from the group consisting of conditions established inside an operational amplifier and conditions established outside the operational amplifier such that the given conditions allow to draw conclusions about a characteristic of undesired properties of the operational amplifier, the characteristic being selected from the group consisting of a presence of the undesired properties and an extent of the undesired properties of the operational amplifier; and
controlling a compensation circuit in dependence of a result of the ascertaining step for performing a function selected from the group consisting of attenuating the undesired properties and eliminating the undesired properties of the operational amplifier.
In other words, the method according to the invention is defined by the fact that conditions which are established inside or outside the operational amplifier and which permit At conclusions to be drawn as to the presence and/or the degree of the undesired properties of the operational amplifier are determined, and thus a compensation circuit which is provided for attenuating or eliminating the undesired properties of the operational amplifier is controlled as a function of the result of the determination.
According to another mode of the invention, the step of controlling the compensation circuit includes at least one step selected from the group consisting of activating circuit elements contained in the compensation circuit, deactivating circuit elements contained in the compensation circuit, changing a connection between circuit elements contained in the compensation circuit and other circuit elements contained in the compensation circuit, and changing a connection between circuit elements contained in the compensation circuit and the operational amplifier.
Another mode of the invention includes the step of actuating switching elements provided in the compensation circuit in order to control the compensation circuit.
Yet another mode of the invention includes the step of controlling the compensation circuit such that the operational amplifier operates in a fast and stable fashion as requested.
A further mode of the invention includes the step of ascertaining whether the operational amplifier is operating in a fast and stable fashion according to the given requirements from at least one response characteristic selected from the group consisting of a duration of a transient response of the operational amplifier and a profile of the transient response of the operational amplifier.
A further mode of the invention includes the step of ascertaining the at least one response characteristic from a ratio of magnitudes of voltages established at input terminals of the operational amplifier.
Another mode of the invention includes the step of concluding that the transient response is still occurring if the voltages established at the input terminals of the operational amplifier are not substantially of a same magnitude.
Yet another mode of the invention includes the step of concluding that the transient response has ended if the voltages established at the input terminals of the operational amplifier are substantially of a same magnitude.
A further mode of the invention includes the step of concluding that the operational amplifier is unstable if the voltages established at the input terminals of the operational it amplifier are alternately substantially of a same magnitude and of different magnitudes.
Yet a further mode of the invention includes the step of reducing a capacitance present in the compensation circuit if the duration of the transient response exceeds an upper limiting value.
According to another mode of the invention, the step of reducing the capacitance present in the compensation circuit includes deactivating an activated capacitor.
Yet another mode of the invention includes the step of increasing a capacitance present in the compensation circuit if the operational amplifier is unstable.
A further mode of the invention includes the step of increasing a capacitance present in the compensation circuit if the duration of the transient response drops below a lower limiting value.
According to another mode of the invention, the step of increasing the capacitance includes activating a non-activated capacitor.
Another mode of the invention includes the step of adjusting the compensation circuit during a normal operation of the operational amplifier.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a circuit and a method for attenuating or eliminating undesired properties of an operational amplifier, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.